Conventionally, a centrifugal fan has been used as a fan of an indoor unit of an air conditioner. In the centrifugal fan, when an impeller is rotated by a fan motor, air is sucked into a case of the indoor unit through a suction port of the indoor unit. The sucked air is guided to an air suction port of a shroud of the impeller along an inner circumferential surface of a bell mouth. In the following, a stream of air guided to the air suction port along the inner circumferential surface of the bell mouth is called as a main stream.
The main stream of air is ejected to the outside (in a direction to be away from a rotation axis of the impeller) from the impeller by a plurality of blades arranged circumferentially between a hub and the shroud. A main part of the air ejected from the impeller is blown into the room through a blow-out port of the indoor unit. However, a part of the air ejected from the impeller is refluxed toward the bell mouth through a space between the outer circumferential surface of the shroud and the case within the case of the indoor unit. The refluxed air merges with the main stream while passing through a gap between the outer circumferential surface of the bell mouth and the inner circumferential surface of the shroud. In the following, a stream of air that is refluxed as described above, and merges with the main stream while passing through a gap between the outer circumferential surface of the bell mouth and the inner circumferential surface of the shroud is called as a reflux stream (a leakage stream).
The aforementioned reflux stream has a high air velocity. Therefore, when the reflux stream passing through the gap collides against the front edges of the blades, noise increases. Further, the reflux stream has large fluctuations in air velocity (air velocity is largely fluctuated). Therefore, the pressure generated on the blade surfaces near the reflux stream is likely to be unstable. Fluctuations in pressure on the blade surfaces are a factor of noise increase.
In particular, in a centrifugal fan having a reduced thickness accompanied by reduction of the thickness of an indoor unit, the channel of the main stream is narrowed. However, it is necessary to secure substantially the same volume of the main stream as the volume in an indoor unit in which the thickness is not reduced. In the centrifugal fan having a reduced thickness, the volume of the reflux stream tends to increase. Therefore, the ratio of the reflux stream with respect to the main stream increases. As a result, the influence of the reflux stream on the main stream increases. In view of the above, it is important to suppress the influence by the reflux stream.
Patent Literature 1 proposes a technique for reducing noise by reducing a reflux stream (a leakage stream). The centrifugal fan disclosed in Patent Literature 1 is provided with a plurality of main blades disposed between a hub and a shroud, and a plurality of small blades formed on the outer circumferential surface of the shroud, wherein the camber line of a shroud-side blade element of each of the main blades is concaved toward the pressure surface, or a front-edge side portion of a shroud-side blade element of each of the main blades with respect to the camber line is tilted in the rotating direction. Patent Literature 1 describes that a pressure raising effect by the small blades reduces a pressure difference between the region on the back surface of the shroud and the region of the bell mouth channel. This makes it possible to reduce the flow rate of the reflux stream, and to reduce the air velocity on the shroud side portion of the front-edge-side portion of each of the main blades. Further, Patent Literature 1 describes forming the shape of the main blades as described above allows for the streams to follow the main blades. Patent Literature 1 describes the aforementioned configuration makes it possible to reduce noise.
However, in the configuration of the centrifugal fan disclosed in Patent Literature 1, it may be impossible to sufficiently reduce the volume of the reflux stream, and it may be impossible to obtain a sufficient noise reduction effect. Further, in the configuration of the centrifugal fan disclosed in Patent Literature 1, the weight of the fan may increase by addition of the small blades, and the cost may also increase.